Apparatus for nuclear logging employing sub wall mounted detectors and modular connector assemblies

ABSTRACT

An apparatus for nuclear logging is presented. In accordance with the present invention, nuclear detectors and electronic components are all mounted in chambers within the sub wall with covers being removably attached to the chambers. A single bus for delivering both power and signals extends through the sub wall between either end of the tool. This bus terminates at a modular ring connector positioned on each tool end. This tool construction (including sub wall mounted sensors and electronics, single power and signal bus, and ring connectors) is also well suited for other formation evaluation tools used in measurement-while-drilling applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 511,537 filed Apr. 17,1990 pending.

BACKGROUND OF THE INVENTION

This invention relates generally to borehole logging apparatus andmethods for performing radiation based measurements. More particularly,this invention relates to a logging in real time wherein the improvednuclear logging apparatus comprises a measurement-while-drilling (MWD)tool.

Oil well logging has been known for many years and provides an oil andgas well driller with information about the particular earth formationbeing drilled. In conventional oil well logging, after a well has beendrilled, a probe known as a sonde is lowered into the borehole and usedto determine some characteristic of the formations which the well hastraversed. The probe is typically a hermetically sealed steel cylinderwhich hangs at the end of a long cable which gives mechanical support tothe sonde and provides power to the instrumentation inside the sonde.The cable (which is attached to some sort of mobile laboratory at thesurface) is also the means by which information is sent up to thesurface. It thus becomes possible to measure some parameter of theearth's formations as a function of depth, that is, while the sonde isbeing pulled uphole. Such measurements are normally done in real time(however, these measurements are taken long after the actual drillinghas taken place).

A sonde usually contains some type of source (nuclear, acoustic, orelectrical) which transmits energy into the formation as well as asuitable receiver for detecting the same energy returning from theformation. The present invention relates to logging apparatus whereinthe source emits nuclear energy, and more particularly neutrons. Whenusing this type of source, the source sends out "fast" (high energy)neutrons into the formation. The fast neutrons leaving the source enterthe formation and slow down by losing energy as a result of collisionswith the nuclei of the formation, finally becoming thermalized. Bythermalized, it is meant that, on the average, the neutrons lose as muchenergy as they gain as a result of collisions, that is, they are inthermal equilibrium with the nuclei of the formation. After some timespent diffusing as thermal neutrons, they may be captured by one of theformation nuclei resulting in the emission of a gamma ray. The energy ofthe gamma ray emitted is characteristic of the particular nucleusinvolved. It is in this context that the term "thermal capture gamma-rayspectra" is used. Examples of well logging tools of this type aredisclosed in U.S. Pat. Nos. 3,379,882, 3,662,179, 4,122,338, 4,223,218,4,224,516, 4,267,447, 4,292,518, 4,326,129 and 4,721,853.

Fast neutrons as a probe source are useful for several reasons. Forexample, chemical sources for the fast neutrons such as Am²⁴¹ Be andPu²³⁸ Be are readily available. Fast neutrons also have a reasonabledegree of penetration into matter, and finally most importantly,neutrons can be especially useful for the detection of hydrogen. Tounderstand the effect of hydrogen, it is helpful to use the analogy of agroup of billiard balls in which the neutron and the hydrogen nucleusare balls having essentially the same mass while the nuclei of otherelements in the formation are balls with much larger masses. Thus, if aneutron collides with the nucleus of an element other than hydrogen, itwill generally lose very little energy. If it collides with a hydrogennucleus, because the masses are nearly equal, it can lose all of itsenergy. The ability of a formation to slow down fast neutrons to thermalenergy then depends primarily on the hydrogen density.

With regard to hydrogen density in a formation, two diametricallyopposed situations may be considered. In the first situation, a group offast neutrons leave a source and slow down in a formation free ofhydrogen, and in a second situation, a group of fast neutrons leave asource and slow down in a formation which has a great deal of hydrogenin it. One expects and will find that the neutrons will have gone muchfarther away from the source in the first case than in the second case.As a result of the foregoing, a technique which has been in use in"wireline oil well logging" for more than thirty years is themeasurement of the spatial distribution of slowed down neutrons. Thistechnique is usually described as neutron porosity logging because theporosity of the formation is inferred from the measurement. Here it istacitly assumed that the pores of the formation are filled with eitherwater or oil (an assumption not always true since there may be gas or amix of all three components). It is also assumed that the hydrogendensity for oil and water are equal (that assumption is also notstrictly true, but can be safely assumed for all practical purposes).

In order to construct a neutron porosity sonde which looks at thespatial distribution of slowed down neutrons, one needs a source ofsufficient intensity (for example, 10⁷ neutrons/sec), and a detectorseparated from the source (for example, 15 inches). There needs furtherto be sufficient shielding between the source and detector to keep theradiation coming directly through the sonde to a minimum. Furtherfeatures needed in the sonde involve reducing the response of the sondeto factors other than porosity, such as borehole size, salinity, etc.Evolution in the prior art of this type of sonde has consisted primarilyin changes in the type of detector used. Originally, Geiger counterswith heavy walls were used. These counters did not detect neutrons butrather gamma rays originating in the formation as a result of thermalneutron capture. The gamma rays strike the walls of the counterreleasing photoelectrons which in turn cause ionization which can bedetected by the counter. Although such detectors are very rugged, theysuffer from the disadvantage of not directly counting the slowed downneutrons.

For a thermal or epithermal neutron detector placed at a sufficientlylarge distance, for example, 15 inches from the source, it can be shownthat the count rate of the detector is of the form A exp(-r/L) where Ais some constant which depends on the source-detector distance and thecounting efficiency of the detector, r is the distance between sourceand detector, and L is some parameter which depends on the slowing down(of neutrons) properties of the formation, i.e., the porosity. For aformation containing no hydrogen, L will be relatively large as comparedwith a formation which is quite porous where L will be significantlysmaller.

It is important to note that the transport of fast neutrons through aformation is characterized by three phases: (1) slowing down to thermalenergy; (2) diffusion at thermal energy; and (3) capture by a formationnucleus accompanied by the emission of a characteristic gamma ray by theexcited nucleus. Only the first phase gives information related directlyto the presence of hydrogen.

Since neutrons are not charged particles, their detection presents somespecial problems. The better detectors usually depend on the neutronundergoing some kind of nuclear reaction, one of whose products is inturn an ionizing particle such as an alpha particle. As a result ofimprovements in technology, the single detector neutron sonde using aheavy-walled Geiger counter was modified with the replacement of theGeiger counter by a He³ proportional counter (normal He is He⁴). He³ hasan unusually high capture cross section for thermal neutrons, and thereaction products (ionizing) are a proton and a triton (H³). Aproportional counter is used since it gives good discrimination againstgamma rays.

The single He³ detector neutron sonde (detecting epithermal neutrons)was thereafter replaced by a two detector neutron sonde (detectingthermal neutrons). The two detector sonde was viewed as being lesssensitive to effects of borehole conditions. Thermal detection ofneutrons was chosen because count rates were higher than with epithermaldetection. In this development, the ratio of the count rates of the twodetectors (near and far from the source) are determined. Instead oflooking at the spatial distribution of neutrons, the rate of change ofthe spatial distribution is being observed. A further refinement of thistechnique is to look at the rate of change of the spatial distributionfor epithermal neutrons.

The foregoing description of prior art nuclear formation logging devicerelates primarily to wire line devices wherein the formation evaluationis done after drilling is completed. More recently, a new generation offormation evaluation tools has been developed which evaluate the earthformation without interrupting the drilling of a borehole. These toolsare known as measurement-while-drilling or MWD tools. A typicalcommercial MWD tool (such as is available from Teleco Oilfield Services,Inc., assignee of the present application) may measure such downholeconditions as the so-called weight-on-bit or "WOB" as well as the torqueacting on the bit, the azimuth direction and the angle of inclination ofthe borehole, borehole temperature, mud resistivity and variouscharacteristics of the earth formations penetrated by the bit. Theoutput signals of the various sensors are coupled to circuits whichselectively control a downhole pressure pulse signaller in the tool forsuccessively transmitting and/or recording encoded data signals (i.e,pressure pulses) representative of these real-time measurements throughthe mud stream in the drill string to suitable detecting-and-recordingapparatus at the surface.

It will, of course, be appreciated that MWD tools have been proposedheretofore for providing real-time measurements of differentradioactivity characteristics of earth formations being penetrated bythe drill bit. Since measurement of natural gamma radiation requiresonly a gamma-ray detector and typical circuits to control the signaller,it has not been difficult to provide MWD tools with thatinstrumentation. Conversely, to measure other radioactivitycharacteristics of earth formations, a MWD tool must also have anappropriate source of radiation (e.g , radioactive chemical source) asdescribed above. It is far more difficult to construct a MWD tool ofthis type (which includes a source of radiation). While such tools havebeen disclosed (for example, see U.S. Pat. Nos. 4,814,609 and4,829,176), there is a continuing need for improved MWD tools fornuclear well logging which include nuclear sources.

SUMMARY OF THE INVENTION

In accordance with the present invention, a new and improved MWD toolfor performing neutron logging is provided. The present inventioncomprises a two detector neutron tool. In accordance with an importantfeature of this invention, the detectors incorporate the Li⁶ isotope oflithium (i.e., Li⁶ I crystal or Li⁶ doped glass). The reaction productsresulting from a neutron interacting with Li⁶ are an alpha particle anda triton. The lithium crystal or glass is fixed to the face of aphotomultiplier tube and the light scintillations which occur therein asa result of neutrons interacting with the lithium are. detected and theresultant signal is amplified by the photomultiplier. The lithiumcrystal or glass is wrapped with a reflective material to improve thelight collection for the photomultiplier tube. These detector componentsare all appropriately packaged for reducing vibrational damage.

Heretofore, it has been generally accepted that lithium crystal or glassdetectors were not practical for tools of this type because of problemsassociated with gamma-ray discrimination. In the case of the He³proportional counter, the pulse heights from neutrons are usually anorder of magnitude larger than those arising from gamma rays, makingdiscrimination quite simple. For Li⁶ I and Li⁶ glass, the pulse heightsfrom neutrons and gamma rays are comparable in magnitude. Of the twoscintillators, Li⁶ I is inherently more sensitive to gamma rays becauseof the presence of iodine which is a high Z (atomic number) material.Nonetheless, the choice of Li⁶ glass does not remove the problem ofgamma-ray discrimination. However, in accordance with another importantfeature of this invention, gamma-ray discrimination is accomplishedusing a novel data processing technique. Using this technique, after aspectrum of particle energies has been acquired, a microprocessor willfit an exponential curve to the spectrum that approximates the portionof the spectrum contributed by the gamma rays. After the gammacharacterization is done, the novel software then strips the gamma raysout of the raw spectrum. This is accomplished by subtracting thegamma-ray spectrum from the raw spectrum. As a result of thissubtraction., the gamma peak is now absent and the spectrum containscounts that are due only to neutrons. If the microprocessor integratesthe counts, under the neutron peak, then the resulting summation willyield the total or gross number of neutrons in the spectrum. The actualneutron count rate is then calculated by dividing the gross neutroncount by the time over which spectrum is collected. This calculationwill produce a value whose units are neutrons per second.

This important gamma-ray stripping software technique permits thepractical use of lithium detectors, which as mentioned, have beenpreviously thought to be problematic as a detector in a nuclear welllogging tool. The use of lithium detectors provided substantialadvantages over both prior art He³ detectors and Geiger counters. He³detectors are often incapable of correctly operating in the harshvibrational environment existing during the drilling of an oil well.Although Geiger counters and proportional counters are similar ingeometry, the latter are more fragile because they generally require amuch finer central wire for their operation. Conversely, the lithiumdetectors of this invention are more rugged and can withstand theextremely harsh downhole drilling environment.

The present invention also provides data processing means fordetermining total background gamma counts detected by the detectorassemblies.

The neutron tool of this invention comprises a steel collar section(sub). Power and signal transmission is effected using a single powerand signal bus (e.g., a wire) which runs the length of the tool. Thispower bus terminates at either end of the tool at a modular connectorcomprised of a conductive metal ring housed within an insulator. Allcomponents of the device are mounted in the sub collar wall includingthe radioactive source, detector assemblies and all associatedelectronics. Three compartments or hatches (equipped with removable highpressure hatch covers) are provided inside the wall of the drill collarfor receiving the tool electronics. A first hatch (known as the DetectorHatch) includes the near and far detector assemblies and a signal bufferboard. A second hatch (known as the modular tool interface or MTI Hatch)contains a low voltage power supply (for powering the conventionalelectronic parts) and modem on a MTI board; and a high voltage powersupply for powering the photomultipliers. A third hatch (known as theProcessor Hatch) includes a multichannel analyzer and microprocessor forcollecting and storing spectra over preselected time periods and thenprocessing those spectra to obtain neutron counts and gamma counts.

The mounting of the detector units and other electronic componentswithin the subwall under a removable high pressure hatch cover offersmany advantages over prior art detector mounting methods including easeof installation and removal, accessibility for diagnosis and adjustment,close proximity of detectors to the outside of the tool and the actualformation wall and facilitates placement of shielding around thedetectors.

The nuclear source is loaded in a novel nuclear source container whichis compatible to the environment encountered in downhole MWD drillingand logging. The source container is a rugged unit designed to withstandstresses, pressures and temperatures experienced in downhole oildrilling. It houses a dimensionally small Nuclear Regulatory Commission(NRC) approved logging source and adapts it to large downhole hardwareby means of a closely controlled diameter, length and thread. On theopposite end of the thread which secures it to the logging device is anovel bayonet which is configured to engage and lock the source assemblyinto the receptacle of a novel installation and removal tool. The shankof the threaded end is smaller and thus weaker than the bayonet end toensure the successful removal of the source from the logging tool. Thisnovel bayonet design also ensures that no person without compatibleequipment will be able to handle the source; and that removal of thesource will be fast and safe.

In accordance with still another feature of this invention, thecenterline of the nuclear source is located orthogonal to the axis ofthe tool in a thick walled section of the tool so that the centerline ofthe active portion of the source is approximately in line with the axisof the detectors being used.

The electronics associated with the nuclear logging tool of thisinvention utilize multichannel analysis wherein the input comprises atrain of analog pulses, each corresponding to the absorption of aneutron or gamma ray and wherein the amplified output is observed over aselected time interval and a pulse height distribution is constructedtherefrom. The electronic processing circuitry includes at least twonovel components, namely a programmable gain amplifier (PGA) and a highspeed peak detector.

The primary feature of the PGA is that its gain can be varied andcontrolled digitally (using a digital bus). The PGA both amplifiesdetector pulses and modifies the frequency characteristics of signalsthat enter it. The PGA includes a low pass filter function whichsignificantly improves the signal to noise ratio and preserves systemresolution by limiting the high frequency signal content in each pulse.By limiting the high frequency signal content, pulse amplitudes are moreeasily quantified by the multichannel analysis (MCA) function thusresulting in better quality spectra.

The high speed peak detector receives the output from the PGA and isimportant as it converts a short, transient amplitude into a stable DCsignal which can easily be measured with an A/D converter.

The above-discussed and other features and advantages of the presentinvention will be appreciated and understood by those skilled in the artfrom the following detailed description and drawings.

DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like elements are numbered alikein the several FIGURES:

FIG. 1 is a diagrammatic view of a neutron porosity tool in accordancewith the present invention;

FIG. 2 is a side elevation view, partly in cross section, of the nuclearlogging tool in accordance with the present invention;

FIG. 3 is a cross sectional elevation view along the line 3--3 of FIG.2;

FIG. 4 is a plan view of the detector, processing and MTI hatches of thetool of FIG. 2 with the hatch covers being removed and with thecircumference of the tool being shown in a single plane;

FIG. 5 is a diagrammatic view of the tool of FIG. 2 including thedetector, processing and MTI hatches;

FIG. 6 is an enlarged view of a modular connector junction hatch in thetool of FIG. 2;

FIG. 7 is a cross sectional elevation view along the line 7--7 of FIG.2;

FIG. 8 is a cross sectional elevation view along the line 8--8 of FIG.2;

FIG. 9 is a cross sectional elevation view along the line 9--9 of FIG.6;

FIG. 10 is a partial sectional view of the tool of FIG. 2;

FIG. 11 is an end view of the modular connector assembly;

FIGS. 11A, 11B and 11C are cross sectional elevation views along thelines 11A--11A, 11B--11B and 11C--11C, respectively of FIG. 11;

FIG. 11D is a side elevation view, partly in cross-section, showing thetool of FIG. 2 connected to another modular tool;

FIG. 11E is an enlarged side elevation view of a portion of FIG. 11D;

FIG. 12 is a side elevation view, partly in cross-section, of thenuclear source container for use in the present invention;

FIG. 12A is a view along the line 12A--12A of FIG. 12;

FIG. 13 is a left end view of the source container of FIG. 12;

FIG. 14 is a cross-sectional elevation view of a nuclear source handlingtool in accordance with this invention;

FIG. 14A is an end view along the line 14A--14A of FIG. 14;

FIG. 15 is an enlarged view depicting the source container of FIG. 12detachably connected to the handling tool of FIG. 14;

FIG. 16 is a diagrammatic view of the electronics and pulse formsassociated with the near and far detectors;

FIG. 17 is a cross-sectional view of a detector assembly;

FIG. 18 is a typical spectrum for an Li⁶ glass scintillator;

FIG. 19A is a graph depicting the fitting of an exponential function tothe gamma-ray portion of the spectrum of FIG. 18;

FIG. 19B is a graph depicting the spectrum of FIG. 18 subsequent tosubtraction of the exponential function in accordance with thegamma-stripping technique of the present invention;

FIG. 20 is a block diagram of the detector hatch electronics depicted inFIG. 4;

FIG. 21 is a diagrammatic plan view of a circuit board for the detectorelectronics;

FIG. 22 is an electrical schematic of the circuit board of FIG. 21;

FIG. 23A is a graph showing a typical neutron detector output pulse;

FIG. 23B is a graph depicting a typical programmable gain amplifieroutput pulse;

FIG. 24 is a block diagram depicting the processor hatch electronics;

FIGS. 25A-C are electrical schematic diagrams of the processor hatchelectronics;

FIG. 26 is a diagrammatic view depicting the multichannel analysisfunction of the present invention;

FIG. 27 is an electrical schematic of the programmable gain amplifierused in accordance with the present invention;

FIGS. 28A and 28B are a pair of graphs depicting the input and outputfor the peak detector function used in the present invention;

FIG. 28C is a timing diagram for the peak detector circuit;

FIG. 29 is a timing diagram for pulse acquisitions made by the detectorsand electronics of the present invention.

FIGS. 30A-C is a flow chart of the digital processing technique forgamma ray stripping; and

FIGS. 31A-B are electrical schematic diagrams of the electronics in theMTI hatch.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, a diagram of the basic components for aneutron porosity tool 10 in accordance with the present invention isshown. This tool comprises a drill collar which contains a neutronsource 12 and two spaced neutron detector assemblies 14 and 16. Allthree components are placed along a single axis that has been locatedparallel to the axis of the tool. The detector closest to the neutronsource will be referred to as the "near detector" and the one furthestaway is referred to as the "far detector".

The tool 10 is placed into service by loading it with a sealed chemicalsource (typically a 5 Curie Americium Beryllium) and lowering it into aformation. Fast neutrons (approximate average 4.4 MeV) are continuouslyemitted by the source and these propagate out into the formation. Thefast neutrons interact with the formation and are slowed down(thermalized) by hydrogen that is present in the environment surroundingthe tool.

Most of the neutrons that are emitted by the source are thermalized andabsorbed by the formation surrounding the tool. Some of the remainingthermal neutrons will then get counted by either the near or fardetectors and contribute to the data collected by the tool.

Calibration of the tool is accomplished by the use of laboratoryformations. These specially built formations allow the tool response tobe characterized for various porosities, borehole size and lithologies.In any given laboratory formation the basic measurement that is takenfrom the tool is the ratio. The ratio is computed by dividing the nearcount rate by the far count rate. After the ratio has beencharacterized, in all of the laboratory formations, it is possible togenerate calibration curves. These calibration curves translate thetools ratio into the porosity of a formation being logged.

Still referring to FIG. 1, tool 10 is preferably associated with ameasurement-while-drilling (MWD) system and comprises a sub section of adrill string 18 which terminates at a drill bit 20. Drillstring 18 hasan open internal diameter 22 in which drilling mud flows from thesurface, through the drillstring and out of the drill bit. Drillcuttings produced by the operation of drill bit 20 are carried away by amud stream rising up through the free annular space 24 between thedrillstring and the wall of the well. The mud column in drillstring 18may also serve as the transmission medium for carrying signals ofdownhole parameters to the surface. This signal transmission isaccomplished by the well known technique of mud pulse generation wherebypressure pulses are generated in the mud column in drillstring 18representative of sensed parameters down the well. The drillingparameters are sensed in a sensor unit in a drill collar 26 near oradjacent to the drill bit. Pressure pulses are established in the mudstream within drillstring 18, and these pressure pulses are received bya pressure transducer and then transmitted to a signal receiving unitwhich may record, display and/or perform computations on the signals toprovide information of various conditions down the well. The method andapparatus for this mud pulse telemetry is described in more detail inU.S. Pat. Nos. 3,982,431, 4,013,945 and 4,021,774, all of which areassigned to the assignee hereof and fully incorporated herein byreference.

Tool Construction and Mounting of Electronic Components and Detectors

Referring now to FIGS. 2-5, the nuclear logging tool of this inventionis shown generally at 10 in FIG. 2 and diagrammatically at 10 in FIG. 5.It will be appreciated that FIG. 4 is a depiction of an entirecircumference of the central portion of tool 10 shown in a single plane.The tool comprises a steel drill collar sub 27 having an axial opening28 along the length thereof. As best shown in FIGS. 3 and 4, sub 27includes three equispaced chambers or hatches 30, 32 and 34 for housingthe tool electronics and detectors. Hatch 30 will be referred to as the"detector" hatch; hatch 32 will be referred to as the "processing" hatchand hatch 34 will be referred to as the "modular tool interface" or"MTI" hatch. The hatches 30, 32, 34 are machined pockets and eachincludes a precision surface 36, 38 and 40 to permit the formation of ahigh pressure seal with a hatch cover 42, 44 and 46, respectively (withthe hatch covers being removed in FIG. 4). A groove is provided in eachhatch cover 42, 44 and 46 for receiving a suitable high pressure sealingmeans such as O-ring 48. It will be appreciated that each surface 36, 38and 40 acts both as the drilling fluid sealing surfaces as well as aload bearing surface for each respective hatch cover. As shown in FIG.2, each hatch cover 42, 44 and 46 is secured to a respective surface 36,38 and 40 by high tensile strength, corrosion resistant bolts 50 ofsufficient size and number (preferably 22) to maintain seal integrityover a wide range of downhole conditions including pressure,temperature, torsion and bending. Hatch 30 is interconnected with hatch32 by a passageway 52 through the subwall 27. Similarly, hatch 32 isconnected to hatch 34 by a passageway 54 and hatch 34 is connected tohatch 30 by a passageway 56 (see FIG. 4). The central portion 29 of tool10 which includes hatches 30, 32 and 34 has an increased diameterrelative to the opposed ends of tool 10. For example, if the ends oftool 10 have a 63/4" outer diameter (OD), central portion 28 may have a71/2" OD. Within this enlarged central portion 28 and between each hatchcover 30, 32 and 34 is a longitudinal passageway 56. Passageways 56provide increased flow of drilling fluid between tool 10 and theborewall.

Referring to FIGS. 2, 4, 6, 7 and 9, sub 27 also includes an upholejunction hatch 58 (FIG. 2) and a downhole junction hatch 60 (FIG. 6). Asin the previously discussed hatches, each junction hatch 58, 60 isassociated with a junction hatch cover 62, 64, respectively. Cover 62utilizes an O-ring 66 to form a high pressure fluid tight seal with aflat surface 72 surrounding hatch 58 (FIG. 7). Similarly, junction hatchcover 64 utilizes an O-ring 70 to form a high pressure fluid tight sealwith a flat surface 68 surrounding hatch 60 (FIG. 9). As will bediscussed below, each junction hatch 58, 60 provides a chamber foreffecting an electrical connection between the electronics disposed inhatches 30, 32 and 34 and a modular connector provided on either end oftool 10. In addition, each junction hatch 58, 60 serves as a pressurebulkhead so that in the event of a failure (e.g. leak) of the modularconnector bus (described below), drilling fluid will be precluded fromflowing into hatches 30, 32 or 34.

The thick walled sub 27 is the structural portion of tool 10 whichtransmits torque and weight to the lower portion of the drillstring andthe method of mounting the detector units and electronic componentswithin the thick subwall 27 is an important feature of this invention.Mounting of the detector assemblies and other electronics within hatches30, 32 and 34 under a removable high pressure hatch cover 42, 44 and 46,respectively provides many features and advantages including ease ofinstallation and removal of components from within the hatches and easeof accessibility to the detectors and electronic components fordiagnosis and adjustment. Also, the use of chambers 30, 32 and 34permits the positioning of the detectors (identified at 74 and 76 inchamber 30 in FIG. 4) as close as possible to both the outside of thetool and the formation wall.

Tool Power and Communications Bus and Modular Tool Interface HatchElectronics

As mentioned briefly above, the nuclear logging tool 10 of thisinvention utilizes a bus which comprises a single wire (see item 78 inFIGS. 2 and 5) that runs the full length of the tool sub 27 through alongitudinal bore 80 (parallel to the centerline of the tool) and isused to supply both power and communications to all locations in thetool. A power return is established for the system by using the steeldrill collar 27 (that composes the body of tool 10) as the common returnpath and system ground.

A significant feature of tool 10 is its ability to be used in a modularsystem. The construction of sub 27 discussed above (including the powerand communications bus 78 and hatches 30, 32 and 34) lends itself foruse, not only as a neutron porosity device, but also in otherapplications such as a gamma density tool or other downhole MWD tools.Accordingly, each end of tool 10 is designed to create what is known asa "modular tool connector". Referring to FIGS. 2, 11 and 11A-E, when thepin 82 and box 84 of two adjacent modular tools are mated, ringconnectors 86 (of the type disclosed in U.S. Pat. No. 3,696,332) insidethe joint establish continuity for the modular tool bus. The powerreturns of the two tools are also joined because the pin and box threadsare making electrical contact (see FIG. 11D). Thus, the tool is a twoconductor system created by the bus 80 (FIG. 2) and drill collar 27(FIG. 3).

As best shown in FIGS. 11 and 11A-E, the modular connection comprisesmetal ring 86 (see also FIG. 2) which is surrounded on three sides by acylindrical section of electrically insulating material 88. An adhesive90 is provided to bond ring 86 to insulator 88. A metal ring86/insulator 88 assembly 89 is provided in a cylindrical groove 92 ineach end of tool 10 and bonded therein by an elastic adhesive (see FIG.2). The connectors are gauged so that they protrude slightly from theshoulder surface to insure positive electrical contact when the twomating rotary connections are torqued up (see FIG. 11D). As shown inFIGS. 11A-C, a rubber tube 91 is bonded on the assembly 89 andpositioned in groove 92 so that the assembly will be preloaded so as tourge assembly 89 outwardly of the end of tool 10 (thus insuring goodelectrical connection to an adjacent sub as shown in FIG. 11D). As shownin FIG. 11B, insulator 88 includes an integral non-rotation lug 93 whichis received in sub 26 for preventing rotation of assembly 89. Also, ring86 includes an integral non-rotation lug 95 which prevents rotation ofring 86. It will be appreciated that non rotation of assembly 89 isimportant in order to prevent breakage of wire 78. Extending from alocation in groove 92 is longitudinal bore 80. At the juncture betweenbore 80 and groove 92 is a connector 94 which effects electricalconnection between wire 78 and ring 86.

Referring to FIG. 11D, the pin end of the modular connection has severalmodifications to a standard API connection. The first is an undercutfillet 101 within the shoulder of the connection. The second is anextended neck 103 on the pin. These two features increase fatigue lifeand improve the bearing stress distribution along the shoulder of therotary connection. This improved loading distribution is needed toeffect a metal to metal seal and prevent the drilling fluids from comingin contact with the connector assembly.

Lubrication for the modular connection system of this invention isaccomplished with the use of a 0.0001 to 0.001 inch thick copper plating(see item 103 in FIG. 11E) and the use of a high shear strength, hightemperature, non electrically conductive lubricant 105. The copperplating is applied to the face and threads of the pin connection. Theplating is required to prevent galling of the rotary connection when ametal based lubricant (typically used on non magnetic drill collars andcomponents) can not be used (it would short the electrical connector tocase).

Both the box connection and pin connection have a circumferential groove107 located in the shoulder of the connection. The location of thegroove is positioned to provide adequate sealing capability againstdownhole hydrostatic mud pressures while subjected to drilling loads(bending, weight on bit, etc.). Within this groove are the severallongitudinally gun drilled holes 80, 80' that receive anti rotationalanchors 93 for the electrical connector assembly and also act as aconduit for the bus wire to travel into the junction box located inboardof the sub. These holes are drilled with great locational accuracy toallow for rotary connection rework and/or recuts which will result inintersecting the newly machined connector groove to these holes.

It will thus be appreciated that if either end of tool 10 is damaged,the sub 26 can be cut off to the damage point with a new groove 92 andring 86 provided at the cut section (along with remachined threading) topermit re-use of the tool.

It will also be appreciated that wire bus 78 is connected to theelectronics in hatches 30, 32 and 34 via electrical connectors 96 (FIGS.4, 8), 98 (FIGS. 4, 9) in junction hatches 58 and 60, respectively. Asshown in FIG. 4, bore 80 extends between both junction hatches 58, 60and modular tool interface hatch 34.

FIG. 5 depicts a system level view of the electronics for the modularneutron porosity tool of this invention As described above, theelectronics have been partitioned so that they fit inside the threehatches 30, 32 and 34. Still referring to FIG. 5, a first function thatthe modular tool bus fulfills is to supply power to any tool that isconnected to the system. A power supply associated with themeasurement-while-drilling electronics package (in sub 26 of FIG. 1)produces a nominal 30 volt DC signal which is supplied to the modulartool bus 78. Circuitry inside the modular tool interface (MTI) hatch 34takes this 30 volt DC signal and converts it into +5 v, +15 v and -15 vsupplies. The specific electronic configuration of the powersupply/interface circuit or MTI board shown at 100 in FIG. 4 is depictedin the electrical schematic diagrams of FIGS. 31A and 31B. Theelectronic components in FIGS. 31A-B are identified in Table 1.

A second function of the modular tool bus is to allow bidirectionalcommunication between all members of the bus. A member can be thought ofas any intelligent piece of hardware which listens or talks over thebus. Because the bus has multiple members, a software protocol is usedwhich prevents more than one member from talking at the same time. Iftwo members attempted to talk simultaneously, a conflict will occur andmost of the communications will probably be corrupted. Thus, when onemember is talking, all of the other members must be listening.

The talking and listening functions are implemented over the bus viafrequency modulation of a 1/2 volt peak to peak sine wave. The sine waveis added to the 30 V DC power signal and when observed on anoscilloscope will appear to be riding on a positive 30 volt DC offset. A273 KHz signal represents a digital "one" and a 245 KHz signalrepresents a digital "zero". The hardware that encodes and decodes thesefrequency modulated signals is located inside hatch 34 on the single MTIboard 100 the electrical schematic of FIGS. 31A-B. It will thus beappreciated that the MTI board 100 may be considered as a power supplyand modem combined into one unit.

                  TABLE 1                                                         ______________________________________                                        COMPONENT    DESCRIPTION                                                      ______________________________________                                        U1           Microcontroller Reset Function                                   U2           High-Voltage Switchmode Controller                               U3           Voltage Regulator                                                U4           Transceiver                                                      U5           Programmable Logic Device                                        U6           Field Effect Transistor                                          U7           Voltage Comparator                                               U8           Inverter                                                         U9           Digital Phase-Locked-Loop Filter                                  U10         Programmable Timer                                               ______________________________________                                    

Nuclear Source and Mounting thereof in Tool

Referring now to FIGS. 2, 4, 6-9, and 12-15, the nuclear source and themounting thereof in tool 10 will now be described. The nuclear sourcecontainer is shown generally at 102 in FIG. 12. The source container isa rugged unit designed to withstand stresses, pressures and temperaturesexperienced in downhole oil drilling. It houses a small NRC approvedlogging source 104 such as Americium 241/Beryllium and adapts it tolarge downhole hardware by means of a closely controlled diameter,length and thread 106 which secures source 102 to the logging tool andis located on the far right end of container 102. On the opposite end ofthe thread 106 is a bayonet 110 which is designed to engage and lock thesource assembly into the receptacle of an installation and removal toolshown at 112 in FIGS. 14 and 14A. The shank 114 of the threaded end 106is smaller and thus weaker than bayonet 110 to ensure the successfulremoval of source 102 from the logging tool. As described below, thenovel bayonet design also ensures that no one without compatibleequipment will be able to handle the source safely.

As best shown in FIG. 8, source 102 is mounted in an opening 116 throughthe subwall 27. Opening 116 is located on a chord with respect to theouter circumference of tool 10 so that the longitudinal centerline ofthe radioactive portion of nuclear source 102 will be positionedorthogonal to the longitudinal axis of tool 10 within a section ofsubwall 27. In this way, the centerline of source 102 will be inalignment or at least nominally (e.g., substantially) aligned with theaxis of the detectors 74 and 76. Opening 116 includes a larger diameterportion 118 which is sized to receive the head of installation andextraction tool 112 and a smaller diameter section 115 having internalthreading for threadible engagement with threaded end 106 of source 102.Source 102 thus relies on elastic strain (due to torque) of the threadedend 106 as the primary means for securement to subwall 27. As a back-upmeasure, a bolt 117 is provided through an opening 119 (which runs fromthe outer wall of sub 12 and intersects opening 116) and abuts againstthe exterior end of source 102 for further securing retention in sub 27.

As is clear from FIG. 8, source 102 is secured such that it is open tothe mud environment but not subjected to mud flow. The mounting ofsource 102 in subwall 27 allows for quick and easy removal from thetool, particularly in the event of an emergency. Also, the positioningof the source along the centerline of the tool provides optimizesneutron emission into the formation.

Referring to FIGS. 14 and 14A, nuclear source loading tool 112 comprisesan inner tube 118 having a spring actuated rod 120 therein. Rod 120 isbiased at one end by a spring 122 and terminates at a bayonet 124. Alocking pin 126 may be positioned through the center of rod 120 and tube118. In addition, tool 112 includes a pair of handles 125 and 127 whichare colinear with inner tube 118. Finally, a pre-set torque wrench 128is attached to tube 118 so that when locking pin 126 locks rod 120 totube 118, wrench 128 will rotate both tube 118 and rod 120 to torquesource container 102 (which has been attached to bayonet 124) to apreselected torque value.

As best shown in FIGS. 13, 14A and 15, the end of bayonet 110 includes acircular shoulder 130 having a pair of opposed slots 132 therein.Bayonet 124 has a complimentary pair of opposed tangs 134 which aresized so as to be received in slots 132. As the tangs 134 are receivedin slots 132, the spring 122 is biased and tool 112 is rotated clockwiseor counterclockwise so that tangs 134 are held tightly by shoulder 130.

Neutron Detectors

Referring to FIGS. 4, 10, 16 and 17, the "near" and "far" neutrondetectors 74 and 76, respectively are positioned in detector hatch 30.Each detector 74, 76 consists of a piece of Li⁶ enriched scintillatingglass 136 configured as a one inch long cylinder having a 1/2" diameter,which is bonded by an adhesive layer 138 to a photomultiplier 140 ofcomparable diameter. Alternatively, scintillator 136 may contain Li⁶ inthe form of Li⁶ I crystal. Photomultiplier tube 140 terminates at afirst circuit board 142. A second circuit board 144 is spaced fromcircuit board 142 with an array of resistors 146 connected betweencircuit boards 142 and 144. A pair of capacitors 148 and 150 are alsopositioned on circuit board 144. The array of resistors plus thecapacitors are a voltage divider network intended to provide voltages tothe various electrodes of the photomultiplier. Three wires 152, 154 and156 terminate at circuit board 144 with wire 152 being connected to ahigh voltage source 158 (FIG. 4) in hatch 34 (FIG. 5); and wires 154,156 being connected to the processor board in hatch 32 (FIG. 5). Eachdetector assembly 74, 76 is potted in a suitable potting compound 160such as a silicone rubber compound (FIG. 17). Potting compound 160 has aplurality of ribs 162 molded therein to provide for expansion and shockabsorption.

Neutrons and gamma rays traverse glass 136 producing lightscintillations which are converted by the photomultiplier (PMT) 140 intovoltage pulses of various heights. A pulse height analyzer circuit thenproduces a spectrum of the type shown in FIG. 18. This spectrum is thensubjected to further analysis in order to subtract out the gamma-rayportion. A ratio is then taken of the net neutron counts in the neardetector to that in the far detector. This ratio, based on a previouscalibration in the laboratory, can then be related to a porosityprovided the nature of the rock matrix is known.

The electronic pulse that is generated by the scintillator/PMT is oflittle use until it undergoes some analog signal processing. Theamplitude of the pulse is typically quite low and it has a very shortduration. By amplifying the pulse and passing it through a low passfilter, the raw pulse is modified into a form whose amplitude is moreeasily measured. (See FIG. 16 for a graphic representation of the pulseshape after passing through pulse amplifier).

As mentioned, a typical spectrum for Li-6 glass is shown in FIG. 18. Thevertical axis gives the quantity of pulses and the horizontal axis isproportional to the pulse amplitude. Examination of this spectrum showsthat it depicts two parts, namely the gamma-ray background and theneutron peak. The gamma-ray portion occurs because Li⁶ glass is alsosensitive to gamma rays, which are always present in a neutron loggingsituation. The neutron peak is predominantly caused by thermal neutronsinteracting with the glass scintillator.

Heretofore, it has been believed by those skilled in the art that Li⁶glass (or other Li⁶ detectors) would be problematic in a loggingmeasurement because of the difficulty of removing the gamma-raybackground. The present invention has overcome past practical problemsby acquiring real time spectra and subjecting these spectra to digitalprocessing techniques.

As will be discussed in greater detail below, after a spectrum has beenacquired, a microprocessor will fit an exponential curve to the spectrumso that it approximates the portion of the spectrum contributed by thegamma rays. A typical exponential function is shown superimposed on araw spectrum in FIG. 19A. After the gamma characterization is done, itbecomes possible to strip the gamma rays out of the raw spectrum. Thisis accomplished by subtracting the gamma function from the raw spectrum.

The result of this subtraction is shown in FIG. 19B. Note that thegamma-ray background is now absent and the spectrum contains counts thatare due only to neutrons. If the microprocessor integrates the countsunder the neutron peak, then the resulting summation will yield thetotal number of neutrons in the spectrum. It should be noted here thatthe gross neutron count is not the same value as the neutron count rate.The neutron count rate is calculated by dividing the gross neutron countby the time over which a spectrum is collected. This calculation willproduce a value whose units are neutrons per second.

It will be appreciated that while two detector assemblies are preferred,this invention may also be utilized in conjunction with one detectorassembly or greater than two detector assemblies.

Neutron Detector Hatch Electronics

FIG. 20 is a block diagram of the electronics associated with thedetector hatch 30 and positioned on circuit board 166 shown in FIGS. 4and 21, and in the electrical schematic of FIG. 22. All of theelectronic components depicted on FIGS. 21 and 22 are identified in FIG.22 (with components U1 and U2 comprising operational amplifiers). Notethat the near and far detector assemblies are so named because of theirdistance from the neutron source. Each detector has been configured sothat it requires only three connections to make it operational. Byapplying 1.5 KV DC to its high voltage input and grounding the powerreturn, pulses will then appear on the output terminal.

When no neutrons or gamma rays are present, a detector 74, 76 (FIG. 10)will be in a quiescent state and output signal should be resting closeto a ground level. If a neutron or gamma flux is present, randomnegative pulses will be observed at the detector's output. FIG. 23Adepicts a typical output pulse. Most detectors will produce an amplitudedistribution that starts at zero and ends with a maximum ofapproximately one volt.

The 1.5 KV DC that is needed to operate the detectors is supplied by thepreviously discussed high voltage power supply 158 (FIG. 20). Thissupply uses +15 V as its input and produces the fixed high voltageneeded by the system. The output of the HVPS (High Voltage Power Supply)cannot be used directly by the photomultipliers because it containsapproximately one volt of peak to peak noise. If high frequency noise ispresent on the high voltage input of a photomultiplier, it will coupledirectly into the output. This problem is eliminated by first passingthe noisy high voltage through a low pass filter 168 (FIG. 20). Filter168 removes the unwanted noise and clean high voltage is distributed toboth detectors 74, 76.

The circuit board 166 (FIG. 4) which contains the HV filter 168 alsocontains two preamplifiers 170 that are used to adjust the signal gainfrom the near and far detectors. The gain of photomultiplier tubes 140usually varies between units and thus requires a hardware gainnormalization when initially setting up the system. This gain adjustmentassures that the neutron peak occurs in the proper position in thespectra.

A second benefit of the preamplifiers is that they present a high inputimpedance to detectors 74, 76. This is important because the detectorshave a very weak load driving capacity and do not work very well whendriving long signal lines or heavy loads. Thus, by placing thepreamplifiers 170 close to the detectors, signal distortion is minimizedand the preamplifiers drive the required loads.

It will be appreciated that FIG. 20 shows the HVPS 158 as being locatedin the detector hatch. Technically this is incorrect because it isactually located inside the MTI hatch 34 (FIG. 4). This simplificationwas taken to help make the detector hatch electronics moreunderstandable.

Processor Hatch Electronics

FIG. 24 shows a block diagram of the electronics located inside theprocessor hatch 32. These electronics are located on a processor board172 shown in FIG. 4 and in the electrical schematics of FIGS. 25A-C.Individual electronic components depicted on FIGS. 25A-C are identifiedin Table 2. This electronics acquires spectra from the detectors 74, 76and then process the spectra to produce detector count rates. An 80C31microcontroller module 174 (FIG. 24) is used to perform all functionsthat require computations and communications with devices outside thetool.

From a nuclear electronics standpoint, this circuitry implements afunction known as multichannel analysis. The input and output of thisfunction is shown in FIG. 26. The input consists of a train of analogpulses, each corresponding to the absorption of a neutron or gamma-rayby the detector. The function observes the amplifier output for apre-selected length of time (e.g. 30 seconds) and constructs a pulseheight distribution.

The channel number is the means by which the amplitude of a given pulseis described by this function. The channel numbers start at zero and endwith a maximum value determined by the resolution of the system. In apreferred embodiment, the system has eight bits of resolution whichmeans that the channel numbers start at zero and reach a maximum valueof 255. Each channel can be thought of as having its own counter whichis incremented when a pulse falls into a given channel. Thus, thesmallest pulses are counted in channel zero and the largest pulses arecounted in channel 255.

Returning to FIG. 24, it can be seen that the near and far detectorsignals enter the processor board through an analog multiplexer 176. Themultiplexer is necessary because MCA (Multichannel Analyzer) can acquirespectra on only one detector at a time. This limitation is solved bymultiplexing the near and far signals into the single MCA on the board.A drawback of this multiplexing scheme is that some counts in the unusedchannel are lost while it sits idle.

During a typical down hole acquisition cycle that lasts approximately 30seconds, the MCA will spend unequal amounts of time on the near and fardetectors. The acquisitions are interleaved by spending 1s and 5s on thenear and far detectors, respectively. This 17%, 83% duty cycle is thenrepeated until the allocated 30 seconds of collection time has elapsed

The near and far detector signals get unequal collection times due totheir inherently different count rates. Since the near count rate ismuch greater than the far count rate, it is quite easy to get goodstatistics on the near channel. Conversely, the low count rates on thefar detector requires that most of the MCA's collection time be spent onthat channel to get acceptable statistics.

The next functional block in the system is a novel programmable gainamplifier (PGA) 178 (FIG. 24). This is an amplifier whose gain can becontrolled digitally. Besides amplifying detector pulses PGA 178 alsomodifies the frequency characteristics of signals that enter it. Pulsesthat are presented to the PGA have the same shapes as that shown in FIG.23A. In this case, however, the amplitude distribution will now rangefrom zero to approximately -10 volts because of the gain contributed bythe preamplifier.

FIG. 23B shows a typical PGA 178 output pulse. Note that the pulse isstill unipolar but that it is now a positive signal. Another importantfeature of this pulse is that it is much more gently rounded than thewaveform shown in FIG. 23A. The shape of the input waveform has beenmodified by subjecting it to a low pass filter. This attenuates the highfrequency signal content of each pulse and thus accounts for the moregentle waveform at the PGA output.

There are two important reasons to include a low pass filter function inthe PGA. By limiting the high frequency signal content in each pulse,the signal to noise ratio will be significantly improved and the systemresolution preserved. All photomultipliers produce some high frequencynoise and inclusion of the filter function helps the MCA place a pulsein the correct channel.

The second important benefit of the filtering relates to the shape ofthe output waveform. If a comparison is made of FIGS. 23A and 23B, itcan be seen that the peak amplitude of the first waveform would be verydifficult to measure because it only lasts for a few nanoseconds. Thesecond waveform, however, has a peak amplitude that lasts much longerand would therefore be much easier to measure. The ability to preciselyquantify pulse amplitudes is an important feature of this invention asit will have a serious impact on the quality of spectra collected bytool 10.

The gain stage of the PGA has an analog range of zero to 5 that iscontrolled with 6 bits of resolution. When the tool is initially poweredup, the gain stage is set to 2.3 and a short acquisition cycle is doneon each detector to locate the neutron peaks. At the time of toolassembly the preamplifier gains are set such that the neutron peak forboth detectors will occur at channel 140 to 160 with the PGA at thedefault gain of 2.3. By placing the neutron peaks within these limits,(at room temperature) the tool will always be able to locate the neutronpeak of each detector under elevated temperature conditions.

Prior to the start of the first real 30 second acquisition cycle, thegain of the PGA is adjusted to place the neutron peak between channels90 and 110. The neutron peak is kept stabilized within these limits tohelp the processing algorithm produce accurate neutron count rates. Keepin mind that each time the microcontroller switches detectors, thecurrent PGA gain (for the chosen detector) must be written to the PGA.

It is possible to place the neutron peak in a known position since themicrocontroller knows the current PGA gain and current peak location. Bythe use of a simple formula, a new PGA gain can be calculated and usedto drive the neutron peak into the desired limits. Just before the startof each new 30 second acquisition cycle, the neutron peak locations arechecked using spectra from the previous 30 second cycle. If eitherneutron peak is not within the correct limits a new PGA gain will becalculated and used during the upcoming 30 second cycle.

Referring now to FIG. 27, the specific circuitry of PGA 178 will bedescribed. PGA 178 unites several functional sections into a singlesystem. The input to PGA 178 is shown in FIG. 27, Section A. U1 isconfigured as a non-inverting voltage follower that provides unity gain.This gives PGA 178 a high impedance input that will not distort theoutput of the preamplifier driving the PGA.

FIG. 27, Section B, shows the low pass filter of the PGA. This is athird order Bessel filter that has a cutoff frequency of 250 KHz. A lowpass filter is used to attenuate the frequencies above 250 KHz in aninput pulse. By limiting the high frequency signal content of an inputpulse, the signal to noise ratio of the system is improved. This resultsin a more gently rounded waveform at the output of the low pass filter.The Bessel type function was chosen over other filter functions becauseof a special property it exhibits. This filter has a linear phase shiftfor most input frequencies which means that a unipolar input pulsecauses a unipolar output pulse. Most other low pass filters will converta unipolar input into a bipolar output. A bipolar output pulse isundesirable because it increases the deadtime of the system.

The gain of PGA 178 is determined by Sections C and D. Section C can bemodeled as an equivalent resistor feeding the summing junction of U6(FIG. 27). B5, B4, B3, B2, B1 and B0 are the digital inputs that set theresistive network to a specific equivalent resistance. The total gainprovided by U6 is computed by summing the contribution of each bit asshown in the following equation:

    GAIN=B5 (-R16/R4)+B4 (-R16/2R4)+B3 (-R16/4R4)+B2 (-R16/8R4)+B1 (-R16/16R4)+B0 (-R16/32R4)

Where:

R4=R6=R8=R10=R12=R14=R15

R5=R7=R9=R11=R13

R4=2 (R5)

With resistive values shown in Sections C and D (FIG. 27), the gain inthis design can range from 1 to approximately 5. By changing the valuesused in the above equation, it is possible to alter the dynamic range tofit another application. It is also possible to increase gain resolutionby adding more switches and resistors to the summing junction of U6.

Section E of FIG. 27 is an output clamping circuit designed to limit themaximum amplitude that the PGA can produce. This is a useful featuresince nuclear detectors will occasionally produce large output pulsesthat may degrade sensitive electronics. The clamping circuit will beginto function when U6 produces an output pulse whose amplitude is +6.2 Vor greater. At that instant, the Zener diode (D1) opens up and clampsthe output of U6 to +6.2 V. The resultant clamping voltage is thentrimmed to a desired value by the use of R17 and R18. With the values inFIG. 27, clamping will occur (at the output of the PGA) at 5.0 V. If aclamping level higher than 6.2 V is desired, a higher value Zener diodecan be used and different trimming resistors selected. U7 in Section Eis the output driver for the PGA. It uses the same configuration as U1and provides the PGA with a low output impedance. This is importantbecause it allows the PGA to drive other circuits with a minimum ofsignal distortion.

Another useful feature of this amplifier is its ability to block DClevels at its input. This can be important if the preamplifier producessignificant DC offsets. An amplifier that did not block these offsetscould possibly become useless by saturating itself. Increased powerconsumption would also be a detrimental side effect. DC blocking occursat two separate locations inside the PGA using a C-R high pass filter.The first filter is at C6 in FIG. 27. The resistive leg of the filtercan be viewed as the equivalent resistance leading to the summingjunction of U6. The second filter is at C18. The resistive portion ofthe filter is formed by the sum of R17 and R18.

Turning again to FIG. 24, the next functional block on the processorboard is the peak detector 180. Peak detector 180 is depicted within thedashed lines of FIG. 25A. It should be noted here that the peak detectorfunction is not related to the neutron peak found in the spectra. FIGS.28A and B show the input and output of the peak detector 180. Pulsesleaving the PGA 178 output are fed directly into the input of the peakdetector 180. As the input pulse is rising towards its maximumamplitude, the output of the peak detector will track its input. Oncethe input pulse has reached its peak and starts heading downwards, theoutput will stop tracking the input. The output of the peak detector isnow producing a DC voltage whose amplitude is identical to the peakamplitude of the input pulse.

The input pulse can have any amplitude ranging from 200 mV to 5 V. It isthe peak detector's function to capture the peak amplitude of the inputpulse and convert this amplitude into a stable DC voltage as shown inFIG. 28B. The output of the peak detector is then sent to an A/Dconverter where it is digitized.

The input pulse of FIG. 28A cannot be sent directly to an A/D converterbecause the peak amplitude only exists for approximately 100 nS. MostA/D converters are not fast enough to accurately convert a high speedsignal such as this; and requires that their inputs be limited to amaximum slew rate. Thus, the peak detector circuit creates a bridgebetween high speed analog pulses and the speed limitations of A/Dconverters.

To understand the operation of circuit 180, reference is made to FIG.28C which illustrates a normal peak detection cycle. When the input ofthe circuit starts to rise (due to the arrival of a pulse) the highspeed comparator (U10) will sense that the input is greater than theoutput. This causes the comparator output to switch from a logic 0 to alogic 1 which causes switch 1 to be in a closed (conducting) state.

Since switch 1 is now closed it will allow the unity gain follower (U9)to charge the memory capacitor (C57) to the same voltage which ispresent on the input. Another unity gain follower (U12) is used toisolate the memory capacitor from the output of the circuit. Referringto FIG. 28C, it can be seen that the output tracks the input only whilethe input pulse is still rising.

Just after the input pulse reaches its peak amplitude, the input voltagewill be slightly less than the existing output voltage. As shown in FIG.28C, this causes the comparator output (U10) to change from a logic 1 toa logic 0. This in turn causes switch 1 to open (become non conducting)and prevent the memory capacitor (C57) from discharging through U9.Since the charge in the memory capacitor is now isolated from any lowimpedance paths, it will effectively maintain a constant voltage on theoutput of U12.

An important feature of peak detection circuit 180 is a digital inputcalled HLD. The HLD line is used to disable the peak detector after apulse has been captured. If a one volt pulse has already been capturedand a two volt pulse subsequently entered the input, then the one voltoutput would be overwritten by the two volt pulse. This problem iseliminated by changing the HLD signal from a logic 1 to a logic 0 aftera pulse is captures. FIG. 28C shows an appropriate use of the HLD inputAs long as the HLD line remains at a logic 0 the input of the peakdetector will be disabled.

After the A/D has completed its conversion, the system must have a meansto initialize the peak detector so that it can capture another pulse.This function is provided by the use of a digital input called RES. IfRES is set to a logic 0, then switch 2 will close (conducting) and causethe memory capacitor to discharge to ground. FIG. 28C shows the properapplication of the RES input.

In peak detector circuits of this type, it is possible that smallamounts of noise (on either the input or output) can lead to erroneousswitching of the high speed comparator (U10). This undesireableswitching will obviously degrade the accuracy of the circuit sinceswitch 1 will not be in its correct state. Most of this noisesensitivity has been alleviated by passing the comparator inputs througha low pass filter. An RC low pass filter is created for the positive andnegative comparator inputs via R38, C63 and R41, C64, respectively. Thislow pass filter constitutes an important feature of the peak detector ofthis invention.

Another important feature of peak detector 180 is that its output droopstowards a ground state. All peak detectors exhibit a condition known asdroop. This is caused by a small continuous movement of charge into orout of the memory capacitor due to IC bias currents or resistive paths.This will result in a time dependant voltage gain or loss at the outputof the peak detector. If droop occurs in a positive direction, then thepeak detector could gradually build a large enough amplitude (on itsoutput) to lock itself up. This problem is avoided in circuit 180 by theaddition of R24. This large value resistor provides a small leakagecurrent that prevents IC bias currents from placing a net positivecharge into the memory capacitor C57.

In some known peak detector circuits of the type described herein, it ispossible to demonstrate an error condition that can lead to thermalburnout of U9, switch 1 and switch 2. If the RES signal is low while HLDis high, then it is possible to have both switches in a closed(conducting) state. Looking at the unity gain follower output U9, it ispossible to trace a current path through switches 1 and 2 to ground. Ineffect, the IC U9 would be driving a short circuit to ground which coulddestroy it or the switches. The peak detection circuit 180 of thisinvention has taken this possible error condition into account by usingthe RES signal to control not only switch 2 but also switch 1. Referringto FIG. 25A, switch 1 (which is used for charging the memory capacitor)is controlled via the output of gate U22. If the RES signal is low, thenthe gate U22 will always open switch 1 and thus prevent U9 from drivinga short circuit. Thus, an error condition involving the use of the HLDand RES signals will not cause hardware failures in circuit 180.

At this point, reference should be made to FIG. 29 for an explanation ofthe coordination of analog and digital events. When a pulse leaves PGA178 it is also being fed into a comparator 182 as well as peak detector180. If the output of the PGA exceeds 200 mV then the comparator outputsignal (CMP) is asserted low. The falling edge of CMP is a signal to theentire system that a valid pulse has left the output of the PGA. Pulseswhich are below the comparator threshold are ignored by the system andwill not become part of the spectrum.

When a falling edge is detected on CMP, a timer starts operating thattimes out in approximately 2.5 microseconds. After the 2.5 microsecondshas elapsed, the READ and HLD lines are asserted low as shown in FIG.29. The READ line tells the A/D converter 184 to start a conversion onthe analog signal presented to it by peak detector 180.

The HLD line serves three purposes in the present invention. The firstof these is to tell the peak detector 180 that no more pulses are to beallowed into its analog memory. Thus, after the HLD line goes low,subsequent pulses are locked out of peak detector 180.

The second function of the HLD line is to generate an interrupt tomicroprocessor 174. This interrupt will tell the microprocessor that apulse has undergone conversion and is waiting to be read. Since themicroprocessor takes approximately 8 microseconds to vector to itsinterrupt, the A/D conversion 184 will already be complete when theservice routine starts.

The last function that the HLD line performs is to enable and disablethe timer that is associated with the microprocessors interrupt line.Whenever the HLD line is low this timer (which is built into themicroprocessor) is gated off. This timer is necessary because it keepstrack of what is known as live time.

Live time is defined as that quantity of time during which the MCA isnot busy processing pulses. Conversely, dead time is the quantity oftime spent by the MCA actually servicing pulses that come in. Toillustrate live and dead time, assume that the MCA is going to collect aspectrum for 1 second in real time. If each pulse takes 50 microsecondsto service and there are 1,000 events during the 1s acquisition period,then the total dead time is 50 milliseconds. The live time for thisscenario would be 950 milliseconds.

The live time is important when calculating count rates from the neutrondetectors. Processing of spectra yields a gross neutron count which isthen divided by the live time to give the neutron count rate. The unitsof the count rate is neutrons/second.

When the microcontroller 174 starts servicing its interrupt routine thefirst action that it takes is to pull the CNT line low. This output ofthe microcontroller gives the microcontroller control over the pulseacquisition hardware. As long as the CNT line is low no other pulses canget into the pulse acquisition hardware. After the microcontroller isfinished servicing the interrupt routine the last thing it does isrelease this line.

Near the end of the interrupt service routine the microcontroller pulsesthe res line low for approximately 1 microsecond. This line serves twopurposes. It simultaneously resets the analog memory of the peakdetector 180 and it resets the hardware that controls the READ line ofthe A/D converter 184.

During the interrupt service route the microprocessor will update itsspectrum to include the pulse that has just been captured. At thebeginning of the service routine, one of its earliest actions is to readthe data from the A/D converter 184. The data bus of the A/D has beenmapped into the memory of the microprocessor and a simple software readinstruction allows the microprocessor to access the conversion.

After the A/D has been read, the microprocessor uses the 8 bit value asa pointer to the correct channel stored in the microprocessor's memory.The existing count in that channel is incremented by one and the newcount is placed back into memory. This process is repeated many times toacquire enough statistics so that a clear spectrum is created.

If a pulse enters the system while the HLD line is low it will beignored and have no impact on the system. Thus, the system is "dead".FIG. 29 shows that HLD is low for almost the whole pulse acquisitioncycle. If the count rates are very high then the dead time may become asignificant fraction of real acquisition time. It is for this reasonthat an MCA is designed to minimize the dead time for each pulseacquisition cycle.

After a preselected acquisition cycle has been completed themicrocontroller 174 will process both spectra and calculate theresultant count rates. The near and far count rates are then availableto either be stored in the memory tool or the near/far ratio may becalculated and transmitted uphole via the mud pulsing system located indrill string section 26 of FIG. 1. The high resolution logs are producedafter drilling has stopped and the contents of the memory tool are readat the surface.

                  TABLE 2                                                         ______________________________________                                        COMPONENT                                                                     (FIGs. 25A-C)   DESCRIPTION                                                   ______________________________________                                        U1              Multiplexer                                                   U22             NAND Gate                                                     U23             Hybrid PGA                                                     U9, U12        Operational Amplifier                                         U10, U15        Comparator                                                    U11             Switch                                                        U13             A to D Converter                                              U14             Voltage Reference                                             U16             NAND Gate Schmitt trigger                                     U17             Programmable Gate Array                                       U18             Serial ROM                                                    U19             Microcontroller Reset Function                                U20             Transceiver                                                   U21             Microcontroller                                               X1              Hybrid Oscillator                                             ______________________________________                                    

Spectrum Digital Processing

As already mentioned, an important feature of this invention is thespectrum digital processing technique which results in thegamma-stripped spectrum of FIG. 19B. This processing technique and theassociated software will now be described with respect to the flow chartof FIGS. 30A-C.

It will be recalled that the raw spectra of FIG. 18 must undergo dataprocessing to achieve the gamma-stripped spectra of FIG. 19B. The dataprocessing is composed of three parts. Part one involves smoothing theraw spectrum. In part two, the shape of the smoothed spectrum ischaracterized by its two peaks, a valley therebetween and a leastsquares fitting is used to produce the gamma-ray background spectrumcurve (FIG. 19A) which is subtracted from the original spectrum to yieldthe preliminary neutron spectrum (free of gamma rays). In the thirdpart, the neutron spectrum is analyzed statistically in order to modifyit and yield as a final result the total neutron counts.

Part I. Using the SMOOTH routine, the raw spectrum (as in FIG. 18) issmoothed (at 300 in FIG. 30A) by passing an eleven point windowsuccessively through the histogram and averaging each set of elevenpoints. The process is then repeated for the purpose of furthersmoothing (at 302 in FIG. 30A). For example, if channels 1 through 11are being averaged, the average value is inserted at channel 6 (themid-point). The eleven channel window is then moved up one channel andchannels 12 through 22 are averaged with the average being inserted atchannel 17. This process is carried forward through the 256 channels. Toobtain the smoothed value for channel zero, it is assumed that there arechannels -1 through channel -5 which have the same value as channel zeroand the window runs from channel -5 through channel +5. A similarprocedure occurs at channel 256. The results of this routine are thenpassed to the SMOOTH routine for further smoothing of the alreadyprocessed raw spectrum.

Part II. The spectrum having now been smoothed must be characterizedaccording to its shape: its two peaks and the valley between them. Thesearch for the gamma-ray peak is performed using the PEAK routine.Starting at channel 1, a search is performed using a ten channel windowand finding the channel with the highest count in that window. Thesearch is then repeated with the window shifted so that the firstchannel of the window corresponds to the channel with the highest countpreviously found. The process continues until the peak channel numberdoes not change. The peak channel number is referred to as NGAMMA. Withthe determination of the gamma peak channel number, NGAMMA, anintegration of the raw spectral curve from channel 2 though (2*NGAMMA)is performed to determine total background gamma counts. This value is asecondary measurement to the neutron count measurement.

Next, the search for the valley between the gamma-ray peak and theneutron peak is made using the VALLEY routine. Starting at channel(NGAMMA+10), a search is made using the ten channel window and lookingfor the lowest count. When the lowest count is found, the window isshifted so that the first channel of the window corresponds to thislowest count. The process continues until the channel number foundhaving the lowest count does not change and NVALLEY1 is the result. Itshould be noted that because of the shape of the spectrum, the VALLEYroutine would not mistakenly find the valley in the region beyond theneutron peak.

The neutron peak is found by using the PEAK routine in a manner whichfollows. A search is made from (NVALLEY1+10) through channel 256 using a100 channel window to find the channel with the highest count. Thesearch is then repeated with the window shifted so that the firstchannel of the window corresponds to the channel with the highest countpreviously found. This process continues until this peak channel numberdoes not change and the result is NNEUTRON.

Lastly, a search is made for a revised valley between the gamma peak andthe neutron peak. This procedure amounts to a check on the initial valueNVALLEY1 with the result being called NVALLEY. Using a 100 channelwindow, a search is made from channel (NVALLEY1-10) through channel 256for the lowest count. When the lowest count is found, the window isshifted so that the first channel of the window corresponds to thislowest count. The process continues until the channel number having thelowest count no longer changes and NVALLEY is the result.

Having evaluated the shape of the curve which includes both neutron andgamma-ray peaks, it now becomes possible to separate out the portions ofthe curve due to each. To do this, a first estimate of the gamma-raybackground is made using a procedure to be described. It is known fromother considerations that the gamma-ray background will be that of adecaying exponential curve. Such a decaying exponential curve can bewritten in the form

    Y=A(X.sup.B)                                               (1)

where A and B are two parameters to be fitted to this curve by a leastsquare fitting routine and X is the channel number. If logarithms ofboth sides of (1) are taken, then (1) becomes:

    log y=log A+b(log X)                                       (2)

the equation for a straight line. Using a software routine known asLLSFIT (at 304 in FIG. 30B), a least square fit to a straight line ismade such that the line is constrained to pass through the followingfour points:

    N1=NGAMMA+0.1(NVALLEY-NGAMMA)

    N2=NGAMMA+0.9(NVALLEY-NGAMMA)

    N3=NNEUTRON+1.67(NNEUTRON-NVALLEY),

but if N3 is greater than 226, then N3=216 and N4=246. The choice of thefour points is not totally arbitrary but is based on experience andexperiment. The LLSFIT routine 304 replots smoothed spectrum as log(channel) vs. log (counts), and fits a straight line constrained to passthrough points N1, N2, N3, and N4. Linear fit is described with aslope=A and an intercept=B.

The background gamma-ray curve so derived is then subtracted channel bychannel between channels N1 and N4 inclusive from the previouslysmoothed spectrum. This subtraction is performed using a softwareroutine known as BCKGRND (at 306 in FIG. 30B). The resultant curverepresents a neutron-only spectrum. The BCKGRND routine subtracts thefit curve described in LLSFIT from the smoothed spectrum. Neutronspectral curve is the result.

This neutron-only spectrum is now subjected to some more refinedanalysis using a software routine known as STATS (a statistical analysisshown at 308 in FIG. 30B). This routine looks at the neutron-onlyspectrum from channel NNEUTRON-(NNEUTRON-NVALLEY) to channelNNEUTRON+(NNEUTRON-NVALLEY). In other words, it is looking at a regionsymmetric about the neutron peak and determines Neutron counts byintegrating the neutron Spectral curve between channels Start and End.It then calculates the following:

(1) SIGMA, the standard deviation about the neutron peak;

(2) MEAN, the mean channel number, the statistical center of the neutronpeak; and

(3) COUNTS, the total number of neutron counts found by integratingunder the resultant curve (the resultant curve being Gaussian)

Part III. In this part, the first estimate of the gamma-ray backgroundcurve is replaced by a more sophisticated analysis. The values N1through N4 are recalculated making use of the just derived value ofSIGMA (which assumes a Gaussian shape for the neutron peak). The newvalues are as follows:

N1=NNEUTRON-(6×SIGMA), but limited to a minimal value of N1 from theprevious pass;

N2=NNEUTRON-(3×SIGMA), but limited to a maximum value of NVALLEY (notethe assumption that three standard deviations from the mean must be veryclose to a minimum value);

N3=NNEUTRON+(4×SIGMA), but limited to a maximum channel number of 216;and

N4=NNEUTRON+(9×SIGMA), but limited to a maximum channel number of 251.

The new values of N1 through N4 are run through the LLSFIT routine (at310 in FIG. 30C) as before to obtain a more accurate estimate of thegamma-ray background curve. Again, the subtraction routine BCKGRND (at312 in FIG. 30C) is used with the new values of N1 through N4.

The newly derived neutron-only curve is now once again subjected to theSTATS (at 314 in FIG. 30C) routine to calculate MEAN and COUNTS. Indoing so, the SIGMA obtained from the first pass processing is used todetermine the end points of this STATS processing: channelsNNEUTRON-(3×SIGMA) to NNEUTRON+(3×SIGMA).

For each acquisition, the above-described processing is done separatelyfor the near and far detector.

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustrations and not limitation.

What is claimed is:
 1. A nuclear logging apparatus for logging aborehole formation comprising:a drill collar sub having a sub wall andhaving opposed ends; a radioactive source in said sub; at least onedetector assembly in said sub, said detector assembly being spaced fromsaid radioactive source and said detector assembly being positioned todetect radiation resulting from emissions emitted by said source; atleast one chamber in said sub wall; at least one chamber cover, saidchamber cover including attachment means for forming fluid tight andremovable attachment to said at least one chamber; and said detectorassembly being positioned in said at least one chamber.
 2. The apparatusof claim 1 including:a flattened surface formed in said sub wall andsurrounding said at least one chamber, said at least one chamber covermating with said flattened surface.
 3. The apparatus of claim 2wherein:said at least one chamber cover includes an annular groove forreceiving an O-ring, said O-ring forming a fluid tight seal with saidflattened surface.
 4. The apparatus of claim 2 including:a plurality ofspaced bolts about the circumference of said at least one chamber cover,said bolts extending through said flattened surface and removablyattaching said at least one chamber cover to said at least one chamber.5. The apparatus of claim 1 including:a single bus substantially throughthe length of said sub, said bus delivering both power and signals tosaid detector assembly in said at least one chamber.
 6. The apparatus ofclaim 5 wherein said bus comprises a single insulated wire andincluding:a longitudinal bore through at least a portion of said subwall extending from said opposed ends of said sub, said insulated wirebeing positioned in said bore.
 7. The apparatus of claim 6 including:afirst ring connector on one of said opposed ends and a second ringconnector on the other of said opposed ends, said first and second ringconnectors being in electrical communication with said bus.
 8. Theapparatus of claim 5 including:a first ring connector on one of saidopposed ends and a second ring connector on the other of said opposedends, said first and second ring connectors being in electricalcommunication with said bus.
 9. The apparatus of claim 8 wherein:each ofsaid ring connectors is partially surrounded by a insulative ring saidinsulative ring being bonded to annular groove in said sub.
 10. Theapparatus of claim 9 including:first anti-rotation anchor meansextending from said ring connector and into said insulative ring. 11.The apparatus of claim 10 including:second anti-rotation anchor meansextending from said insulative ring and into said sub.
 12. The apparatusof claim 9 including:biasing means in said groove for preloading saidring connector.
 13. The apparatus of claim 5 including:a pair of spacedjunction openings in said sub wall and on either side of said at leastone chamber; a junction opening cover removably attached to eachjunction opening and forming a fluid tight seal therebetween; whereinelectrical connections are made in each of said junction openingsbetween (1) said detector assembly in said at least one chamber and (2)said bus.
 14. The apparatus of claim 13 including:flow prevention meansin said junction openings to prevent fluid from flowing from saidjunction openings and into said at least one chamber.
 15. The apparatusof claim 1 including:a plurality of chambers; and a plurality of chambercovers each removably attached to one of said chambers.
 16. Theapparatus of claim 15 including:three aligned chambers equally spacedabout the outer circumference of said sub.
 17. The apparatus of claim 15wherein:each of said chambers is interconnected by a passageway throughsaid sub wall.
 18. The apparatus of claim 1 wherein said drill collarsub has a longitudinal axis and wherein said sub wall has an outercircumference and including:a first passage through at least a portionof said sub wall, said first passage terminating at a first opening onsaid outer circumference of said sub wall with said radioactive sourcebeing mounted in said first passage.
 19. The apparatus of claim 18wherein:said source is adapted to be exposed to drilling fluids throughsaid first opening.
 20. The apparatus of claim 18 wherein:said source ismounted orthogonal to said longitudinal axis of said sub.
 21. Theapparatus of claim 18 wherein:said source is substantially aligned withsaid at least one detector assembly.
 22. The apparatus of claim 18wherein said source is mounted in a container, said container having afirst end with external threading, and including:threading in said firstpassage wherein said first end of said container is threadably connectedto said threading in said first passage.
 23. The apparatus of claim 18including:a second passage extending between the outer circumference ofsaid sub wall and said first passage; and a bolt in said second passagein engagement with said source.
 24. The apparatus of claim 18wherein:said first passage is positioned on a chord with respect to saidouter circumference of said sub wall.
 25. The apparatus of claim 24wherein:said radioactive source is orthogonal to said longitudinal axisof said sub.
 26. The apparatus of claim 1 wherein:said emissions emittedby said source comprise neutrons.
 27. A formation evaluationmeasurement-while-drilling tool comprising:a drill collar sub having asub wall and having opposed ends; at least one sensor assembly in saidsub; at least one chamber in said sub wall; at least one chamber cover,said chamber cover including attachment means for forming fluid tightand removable attachment to said at least one chamber; said sensorassembly being positioned in said at least one chamber; a single bussubstantially through the length of said sub, said bus delivering bothpower and signal to said sensor assembly in said at least one chamber;and a first ring connector on one of said opposed ends and a second ringconnector on the other of said opposed ends, said first and second ringconnectors being in electrical communication with said bus.